The invention relates generally to the field of electrical switch gear used to supply energizing power to a load. More particularly, the invention relates to a technique for maintaining power applied to such loads during transient losses of power from a grid or other power supply.
A wide range of applications exist for powered loads, particularly in industry, but also in mining applications, material handling applications, marine, commercial, and other fields. In most of these applications power from a source is switched by various switch gear, typically contactors, to apply power to the driven load. The contactors, which may be sometimes referred to as relays of various types, will generally include an electromagnetic operator and a set of moveable contacts which can be moved by energizing the operator. When a control voltage is applied to the operator coil, which essentially forms an electromagnet, an armature is moved by the magnetic flux produced by the coil to open or close the contacts in the device. Opening and closing contacts thus establishes or interrupts a current carrying path through the device to control application of power to the load.
In many applications, the provision of control power is subject to fluctuations in power from the source. That is, relatively small or intermittent loss of power is possible due to fluctuations in the grid, other loads that are switched on and that share the same power source circuitry, and so forth. Certain areas and power networks tend to suffer from such short losses in power on a fairly regular basis. Depending upon the nature of the load and process to which the load is applied, such transient losses of power, which would may be termed “brownouts” can be extremely problematic. For example, driven loads can be caused to stop or start, or circuitry may need to be reset following such common brownouts. The questionable reliability of certain grids and power sources in many locations makes the consistent application of power to loads via contactors and relays particularly problematic, to the point of jeopardizing the ability to reliably drive the process at all.
There is a need, therefore, for techniques that will allow for switch gear to survive short losses in power and maintain power either applied to a load or not applied to a load, depending upon the circuit configuration. There is a particular need for arrangements which will provide for such brownout ride-through while allowing the systems to work with other common circuit configurations, particularly emergency stops that override the ride-through function.